Tire sidewalls are generally constructed of very flexible, rather soft rubber stocks in order to deliver ride comfort and to protect body cords. The present invention does not alter the methods of tire building but employs instead a different, stiffer rubber stock for construction of the sidewalls. Stiffness is increased at least three times over conventional rubber stocks and up to about 200 times or more. Stiffer sidewalls as disclosed herein can be employed in substantially any tire construction, bias ply or radial, and therefore, the invention should not be construed as limited to any particular type.
The rubber from which tire sidewalls are constructed can be natural or synthetic or mixtures thereof alone or with natural rubber. The present invention employs the incorporation of compatible additives to the rubber stock to increase the stiffness. One particularly useful additive is zinc dimethacrylate.
U.S. Pat. No. 3,823,122 provides the addition of a substituted acrylic acid or acid salt to SBR synthetic rubbers to improve oil and solvent resistance. Zinc methacrylate is specifically disclosed in amounts of 2.5 to 5.0 parts per 100 or rubber (phr).
U.S. Pat. No. 4,056,269 is directed toward homogeneous golf balls comprising filled cis-polybutadiene that are highly cross-linked by a salt of divalent metals such as zinc and organic acids such as methacrylic. Amounts added are 20 to 50 phr.
U.S. Pat. No. 4,065,537 is directed toward a process for producing molded golf balls from a molded, cross-linkable elastomer. The latter comprises polybutadiene containing from about 15 to 60 parts per 100 of a salt such as zinc methacrylate.
U.S. Pat. No. 4,082,288 provides the preparation of an adjuvant from zinc oxide and methacrylic acid which is thereafter dried, ground and mixed with elastomers such as polybutadiene in amounts of 10 to 60 phr to form golf balls.
U.S. Pat. No. 4,191,671 is directed toward abrasion and fatigue resistance rubber compositions comprising a diene elastomer and 1.5 to 17 phr of an unpolymerized carboxylic acid or metal salt thereof. Carboxylic acid metal salts include higher unsaturated carboxylics and aryl carboxylics with metals such as zinc.
U.S. Pat. No. 4,192,790 provides for reducing the viscosity of synthetic and natural rubbers by incorporating basic zinc dimethacrylate-type adjuvants into the rubber at 0.1 to 7 phr, and a particulate filler.
U.S. Pat. No. 4,266,772 provides solid golf balls and compositions comprising cis-polybutadiene and 10 to 60 phr of a preformed zinc oxide-methacrylic acid reaction product in proportions of 1 to 1.5 moles of zinc oxide per mole of methacrylic acid. The reaction product is powdered to pass a 200 mesh sieve.
U.S. Pat. No. 4,495,326, owned by the Assignee of record, discloses the use of 0.2 to 15 phr of zinc dimethacrylate in a sulfur-curable rubber composition to improve processing properties in the uncured state and improved strength and dynamic properties in the cured state.
U.S. Pat. No. 4,529,770, also owned by the Assignee of record, is directed toward vulcanizable polymeric compositions comprising natural or synthetic rubber; 2.5 to 20 phr of a zinc dimethacrylate, having a surface area of 3.7 to 5.4 m.sup.2 /g; 30 to 70 phr of a filler and, a peroxide curing agent. These compounds exhibited improved stress-strain properties.
Thus, while the majority of the foregoing patents are directed toward golf balls, several recognize that higher modulus, higher tensile strength and greater elongation can be obtained by the addition of metal salts of unsaturated carboxylic acids to rubber. Nevertheless, in no instance have stiffer rubber stocks been employed as sidewall compositions. The art disclosed herein does not suggest such a usage, nor are pneumatic tires having stiffer sidewalls suggested thereby.